Flat cathode ray tube (CRT) displays include displays which exhibit a large aspect ratio (e.g., 10:1 or greater) with respect to conventional deflected-beam CRT displays, and which display an image in response to electrons striking a light emissive material. The aspect ratio is defined as the ratio of the diagonal length of the display surface to the display thickness. The electrons which strike the light emissive material can be generated by various devices, such as by field emitter cathodes or thermionic cathodes. As used herein, flat CRT displays are referred to as flat panel displays.
Conventional flat panel displays typically include a faceplate structure and a backplate structure which are joined by connecting walls around the periphery of the faceplate and backplate structures. The resulting enclosure is usually held at a vacuum pressure, typically around 1.times.10.sup.-7 torr or less. To prevent collapse of the flat panel display under the vacuum pressure, a plurality of electrically resistive spacers are typically located between the faceplate and backplate structures at a centrally located active region of the flat panel display.
FIG. 1 is a cross sectional and schematic view of a portion of a conventional flat panel display 100. Flat panel display includes faceplate structure 120, backplate structure 130, spacer 140 and high voltage supply 150. Although only one spacer 140 is shown in FIG. 1, it is understood that flat panel display 100 includes similar additional spacers which are not shown.
Faceplate structure 120 includes an insulating faceplate 121 (typically glass) and a light emitting structure 122 formed on an interior surface of the faceplate 121. Light emitting structure 122 typically includes light emissive materials, such as phosphors, which define the active region of the display 100. Light emitting structure 122 also includes an anode (not shown) which is connected to the positive (high voltage) side of voltage supply 150.
Backplate structure 130 includes an insulating backplate 131 and an electron emitting structure 132 located on an interior surface of backplate 131. Electron emitting structure 132 includes a plurality of electron-emitting elements 161-165 which are selectively excited to release electrons. Electron emitting structure 132 is connected to the low voltage side of voltage supply 150. Because light emitting structure 122 is held at a relatively high positive voltage (e.g., 5 kV) with respect to electron emitting structure 132, the electrons released by the electron-emitting elements 161-165 are accelerated toward corresponding light emissive elements on the light emitting structure 122, thereby causing the light emissive elements to emit light which is seen by a viewer at the exterior surface of the faceplate 121 (the "viewing surface").
Spacer 140 is connected between the substantially planar lower surface of light emitting structure 122 and the substantially planar upper surface of electron emitting structure 132. If spacer 140 is made of a uniform material having a constant resistivity, the voltage distribution along spacer 140 is approximately equal to the voltage distribution in free space between electron emitting structure 132 and light emitting structure 122.
FIG. 2 is a cross sectional and schematic diagram of another conventional flat panel display 200. Because flat panel display 200 is similar to flat panel display 100, similar reference elements in flat panel displays 100 and 200 are labeled with similar reference numbers. Flat panel display 200 additionally includes focusing structures 133a-133f. One edge of spacer 140 contacts focusing structure 133a, and the opposite edge of spacer 140 contacts light emitting structure 122.
Focusing structures 133a-133f are electrically connected to the low voltage side of voltage supply 150. As a result, focusing structures 133a-133f assert repulsive forces on the electrons emitted from electron emitting elements 161-165. These repulsive forces tend to direct or focus stray electrons toward the appropriate light emitting elements on light emitting structure 122.
However, combining focusing structures 133a-133f with electron emitting structure 132 results in a substantially non-planar equal potential surface. That is, the upper surface of electron emitting structure 132 and the upper surfaces of focusing structures 133a-133f are at approximately the same potential, e.g., 0 Volts. This non-planar equal potential surface can cause the voltage distribution along spacer 140 to be different from the voltage distribution in free space between electron emitting structure 132 and light emitting structure 122. These unequal voltage distributions can result in undesired deflection of electrons emitted from electron emitting elements adjacent to spacer 140 (e.g., electron emitting elements 161 and 162).
It would therefore be desirable to have a method and structure for locating a spacer between a light emitting structure and a focusing structure which maintains a voltage distribution along the spacer which is equal to the voltage distribution in free space between the electron emitting structure and the light emitting structure.